A need exists for an antireflective treatment that is characterized by extremely low reflectance and high transmittance over a broad wavelength band, even at high angles of incidence. Additionally, it must be cost effective, flexible, and able to cover large areas. Such a solution would be particularly useful as an easily distributed antireflective treatment that may be applied onto glazing materials for the picture frame industry, TV screens, personal handheld devices, cellular phones, shop windows, and other applications.
Optical coatings involving either single or multiple thin layers are well known in the prior art for antireflective purposes. This technique uses destructive interference from the interfaces between materials of different indices of refraction to reduce the overall reflectance. Production of such thin film treatments requires constant effort to maintain coating thickness and composition uniformity, and therefore they are difficult to form on large surfaces. Thin film antireflective coatings (AC) have traditionally been reserved for high-end optics, since they can have excellent optical properties, but have yet to be manufactured cost-effectively. For example, the optical characteristics of a single layer AC are highly sensitive to the wavelength, and have narrow angular acceptance. To get broadband and wide angular acceptance, an antireflective coating with multiple layers should be employed. However, that dramatically increases associated technical problems and significantly raises the cost.
The requirement for inexpensive solutions was understood many decades ago. Adams U.S. Pat. No. 2,348,704 describes a two-step acid etch process used to reduce the reflectivity of glass. The result of the acid etch was the formation of a microscopic network of pores, or skeletonized structure, in a thin layer near the surface of the glass. Precise control of the thickness of this porified layer allowed it to serve as a thin film of intermediate index in analogy to conventional optical coatings. Recently, a similar acid etching method has been proposed by Zuel et al. U.S. Pat. No. 5,120,605 to produce an antireflective treatment on glass that potentially can be used for large areas. It describes a method for a high-throughput treatment that results in a textured glass surface consisting of a distribution of islands and a superimposed porified layer. While relatively cost effective compared to vacuum deposited thin films, such acid etched solutions are limited to rigid glass substrates only and therefore the final product lacks flexibility.
Another class of inexpensive solutions, with potential cost reductions beyond even the acid etching techniques, involves replication of a texture into a plastic. Maffitt et al. U.S. Pat. No. 4,114,983 describes a method for production of a polymeric element with antireflective microstructured surface comprising a four-step process. Those steps include (1) creation of a master surface relief via a two step etch process in glass, (2) galvanic replication on the glass surface to create a durable metal stamper, (3) stamping into a heated thermoplastic material to transfer the surface relief into the plastic, and (4) release of the plastic element from the stamper. Essentially, the Maffitt patent teaches replication of an acid etched antireflective glass texture into a plastic optical element. However, the acid etched porified surface is not ideal for replication because the surface profile may have overhangs and voids that inhibit release, or cusps that break off during release, resulting in loss of fidelity with each replication. This leads to a reduction in the antireflective performance of the replicated polymer.
Polymeric materials not only serve the requirement of cost-effectiveness, but may also be employed to provide flexible antireflective solutions. Allen et al. U.S. Pat. No. 4,333,983 discloses the use of a thin layer of aluminum oxide to form the adhesion layer between a pliable polymer substrate and a thin dielectric optical coating. The result is a flexible antireflective film for applications that do not require very high performance optical coatings. The Allen patent asserts that such a polymer antireflective film can be inexpensively distributed, since a manufacturer wishing to add an antireflection coating to an optical element can simply purchase the film, cut it to desired geometry, and then apply it to the article. With the advent of dual magnetron sputtering technology, as described by J. Strümpfel et al. (40th Annual Tech. Conf. Soc. of Vacuum Coaters, New Orleans, 1997), large area uniform coatings on polymer webs are possible. However, this technology attains antireflectivity in the same manner as thin film optical coatings, and therefore is still subject to limitations in band performance and angular acceptance. In addition, mechanical integrity due to cracking and separation of layers remains an issue, as well as the cost of manufacturing.
Rather than providing a deposited or sputtered optical coating, the polymer film may be provided with a microtextured surface to achieve the antireflective properties. Schroeder et al. U.S. Pat. No. 5,820,957 discloses an optically transparent polymeric film with textured surface, and an adhesive on the backside. The textured surface functions to diffuse incident light to a degree sufficient to reduce specular gloss. This patent describes a process of very inexpensive replication, potentially much cheaper than magnetron sputtering. To be specific, the Shroeder patent describes an antiglare texture, but the described replication principle can also be applied to antireflective surfaces. That is, the textured film of the patent receives the texturing by casting, imprinting, or embossing from a previously textured master. Thus the properties of the master texture are essential to the optical behavior of the replicated film. This leads back to the problem inherent in the Maffitt patent, i.e. what is a desirable surface profile for the antireflective master to obtain high optical quality and efficient replication?
Clapham and Hutley U.S. Pat. No. 4,013,465 describes a method to produce antireflective textured surfaces that are broadband with large angular acceptance. The microtexture is characterized by a surface covered with a regular array of conical protuberances, where the feature sizes of the tapered protuberances are in general sub-wavelength. Such surface profiles are known in the art as “moth-eye” antireflective surfaces, since Bernhard (Endeavor 16, p. 76-84, 1967) first noticed that the eyes of night flying moths were covered with an array of sub-wavelength protuberances, and hypothesized that the function of this profile was precisely to reduce the reflectivity of the eyes of these moths making them less detectable to predators. The moth-eye microtexture acts effectively as a gradient index layer, and therefore has excellent antireflective properties when compared to multilayer thin-film coatings.
The moth-eye profile has an advantage over other textured surfaces, such as those produced by acid etching, in that it possesses a very smooth profile free of overhangs, voids, or cusps that could lead to degradation during the release phase of polymer replication. Thus a single moth-eye master can generate a multitude of daughter surfaces with very little loss of fidelity. Despite all the benefits of the moth-eye antireflective surface, a reliable and cost effective method for producing moth-eye textures over large areas with high uniformity has not been developed.
The Clapham and Hutley patent suggests a photo-exposure method to produce the moth-eye microtexture. The patent further discloses the specific technique of interference lithography that involves interfering multiple beams of coherent light, to create the moth-eye profile. The distinct benefit of this technique is that it yields relatively high contrast sub-wavelength intensity variations. However, it is known that interference lithography has some drawbacks. For example, the optical system that generates the interference pattern must be very stable in space and time. Difficulty in meeting the requirement of stability leads to reduction in the reproducible yield of high quality exposures. A more serious drawback is the difficulty with achieving large areas of spatial uniformity in the pattern, as any non-uniformity in the spatial distribution of the light intensity of the laser beams will be recorded in the pattern. Therefore, there is a need for another technique that is capable of fabricating microtextured surfaces which are similar to the moth-eye patterns produced by interference lithography, but have greater uniformity and are more amenable for scale-up to large areas.
Another limitation of interference lithography is the strict periodicity of the pattern generated by this technique. In order to add arbitrary non-periodic features to the surface profile, additional fabrication steps must be introduced to the process. Gombert et al. U.S. Pat. No. 6,359,735 describes a two-step process that combines a periodic moth-eye pattern with a randomized rough surface. In this patent, the periodic moth-eye pattern is written using interference lithography, while the non-periodic portion is achieved with a separate step, such as sand blasting, mechanical grinding, or exposure of photoresist to a laser speckle pattern. A new method for creating antireflective microtextured patterns that was not limited to strictly periodic patterns would simplify this process. This new method might also enable the fabrication of quasiperiodic patterns whereby the distribution of protuberances do not fit on a perfectly regular grid, but deviate from periodicity in a subtle, but precise way. A quasiperiodic microtextured surface may have desirable optical properties distinct from either periodic or completely random microtextured surfaces.
A new technique is proposed in this invention for the fabrication of antircflective surfaces. This technique enables the production of microtextures that have the following features: effective antireflectivity over a broad wavelength range, wide angular acceptance of incident light, pliability, ease of patterning non-periodic or quasiperiodic features, ease of scalability to large areas, high manufacturing yield, and low production cost.